Glow discharge deposition is employed for the preparation of thin films of a variety of materials such as semiconductor materials, electrical insulators, optical coatings, polymers and the like. In a typical glow discharge deposition, a process gas, which includes a precursor of the material being deposited, is introduced into a deposition chamber, usually at subatmospheric pressure. Electromagnetic energy, either AC or DC, is introduced into the chamber and energizes the process gas so as to create an excited plasma therefrom. The plasma decomposes the precursor material and deposits a coating on a substrate maintained near or in the plasma region. Frequently, the substrate is heated to facilitate growth of the deposit thereupon. This technology is well known in the art.
In many glow discharge deposition processes, the rate of deposition is fairly slow, typically on the order of 2-10 angstroms per second. This low deposition rate can be a drawback in commercial applications, particularly where relatively thick layers of material are being prepared. Thin film photovoltaic devices generally include a relatively thick layer of intrinsic semiconductor material disposed between oppositely doped semiconductor layers. This intrinsic layer may be on the order of many thousands of angstroms thick. Similarly, electrophotographic photoreceptors are often manufactured from amorphous silicon alloys, and typically include layers which may be several microns thick. Low deposition rate processes are clearly unattractive for applications such as these, and a number of higher speed processes have been developed which can deposit layers at rates of 10-200 angstroms per second. In some instances, these high deposition rate processes employ microwave energy to create the plasma, whereas in other instances other frequencies of electromagnetic energy are employed.
One very important class of semiconductor materials which are manufactured by plasma deposition processes are the Group IV semiconductor alloys. Most typically, these materials comprise alloys of silicon and/or germanium together with alloying, modifying and dopant elements, the most typical of which are hydrogen, halogens, and the Group III and Group V elements. Within the context of this application, these materials will be generally described as hydrogenated, Group IV semiconductor alloys and will include amorphous, microcrystalline, crystalline and polycrystalline materials. It has generally been found that for particular electronic applications, hydrogenated Group IV alloys which have been deposited at relatively high rates have electronic properties which are somewhat inferior to those of materials deposited at lower rates. It is speculated that this is the result of several factors. It is believed that materials deposited at higher rates frequently include undesirable morphologies, such as deviant bonds, broken bonds, strained bonds, vacancies and the like, and these defects can affect the transport properties of the materials. Additionally, it has been found that Group IV semiconductor alloys deposited at high rates tend to incorporate more hydrogen than do comparable materials prepared under lower deposition rate conditions. Hydrogen content is a particularly important parameter for the semiconductor alloys, since hydrogen tends to increase the band gap of the materials thereby changing their optical and electronic properties. If these high-gap materials are incorporated into photovoltaic devices, it has been found that the increased hydrogen content will decrease the short-circuit current of the cell (Jsc) and increase the open-circuit voltage (Voc) of the device. Generally, it has been found that photovoltaic devices which include Group IV semiconductor layers prepared in accord with prior art high-deposition rate processes have efficiencies which are lower than the efficiencies of similar devices prepared by low deposition rate processes. It is speculated that this is due both to the effect of hydrogen on the band gap and the effect of undesirable morphologies.
As noted above, the substrate in a glow discharge deposition process is typically heated to facilitate growth of the deposit; and in accord with the present invention, it has been found that substrate temperature is a parameter which has a direct influence upon the quality of the deposited semiconductor material and hence the efficiency of photovoltaic devices manufactured therefrom. Heretofore, the art has paid little attention to the parameter of substrate temperature. In low deposition rate processes of the prior art, substrate temperatures in the range of 500.degree.-600.degree. K. (227.degree.-327.degree. C.) have been found to provide quality deposits of hydrogenated Group IV semiconductor alloys. When the art turned to the use of high-deposition rate processes many parameters, including substrate temperature, were maintained constant. For example, U.S. Pat. Nos. 4,504,518; 4,517,223 and 4,701,343 all describe high-deposition rate, microwave energized, glow discharge deposition processes. These patents broadly recite that the depositions may be carried out at a substrate temperature in the range of 20.degree. C. to 400.degree. C. and that a preferred substrate temperature range is 250.degree.-325.degree. C. U.S. Pat. No. 4,515,107 shows the preparation of silicon alloy materials by a microwave process employing substrate temperatures of 350.degree. C.; U.S. Pat. No. 4,713,309 describes the manufacture of silicon photoreceptors in a microwave energized process carried out at substrate temperatures of approximately 225.degree. C.
While the prior art describes a number of high deposition rate processes carried out at several different substrate temperatures, the prior art has not recognized that substrate temperature is a parameter which must be controlled with specific regard to deposition rate, in order to provide high quality semiconductor materials. The prior art, in fact, teaches away from this important principle of the present invention. U.S. Pat. No. 4,713,309 discloses the deposition of hydrogenated Group IV semiconductor alloys at rates of 20, 40, and 100 angstroms per second, and throughout the experimental series, the substrate temperature was maintained at a constant 300.degree. C. A similar teaching is found in U.S. Pat. No. 4,721,663 wherein a series of depositions were carried out at various rates ranging from 20-200 angstroms per second, all with a constant substrate temperature of 300.degree. C. U.S. Pat. No. 5,114,770 discloses a series of high rate depositions of hydrogenated silicon alloys, typically in the range of 100 angstroms per second, utilizing substrate temperatures of 180.degree.-280.degree. C. In this series of depositions, there is no correlation made between substrate temperature and deposition rate. Furthermore, in one of the experiments, the substrate temperature inadvertently exceeded 350.degree. C., and it was reported that the semiconductor layer delaminated from the substrate under these conditions.
Thus, it will be appreciated that there is a need for a method wherein high quality layers of hydrogenated, Group IV semiconductor alloys may be prepared at high rates. It will also be appreciated that the prior art has not been able to provide such a process, and that the prior art has not recognized that substrate temperature, in a glow discharge deposition process, is a parameter which must be controlled as a function of deposition rate. The present invention provides an improved process for the deposition of hydrogenated Group IV semiconductor alloys by maintaining the substrate at a preselected temperature which is positively correlated with the deposition rate. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.